By Kim Hyun SEOUL, Nov. 10 (Yonhap) -- North Korea demanded an apology from South Korea over a naval clash on the western sea border on Tuesday, calling the skirmish a "grave armed provocation" by the South that targeted a patrol boat on routine duty.

The two-minute skirmish occurred near Baekryeong Island shortly before midday. South Korea sustained no casualties, while the North Korean patrol boat retreated after apparently suffering "considerable" damage, according to the South Korean Navy. The extent of possible casualties in the North was not immediately known.

The Supreme Command of the North's Korean People's Army (KPA) accused South Korea of chasing and opening fire on the North Korean patrol boat, which it said was on routine duty tracing an unknown object that had intruded into its territorial waters.

"When the patrol boat was sailing back after confirming the object at about 11:20 (a.m.), a group of warships of the South Korean forces chased it and perpetrated such a grave provocation as firing at it," the command said in a report carried by the North's Korean Central News Agency (KCNA).

The report did not say what the object was found to be.

The South Korean military said the North Korean boat had come 2.2km south of the Northern Limit Line, the de facto border in the Yellow Sea, when South Korea started sending warning signals. But as the boat continued sailing southward, South Korea fired a warning shot that was followed by the exchange of more shots from both sides.

Bloody skirmishes occurred at the Yellow Sea border in 1999 and 2002, claiming scores of lives on both sides.

"The patrol boat of the north side, which has been always combat-ready, lost no time to deal a prompt retaliatory blow at the provokers," the command said, according to the English-language KCNA report.

"Much flurried by this, the group of warships of the South Korean forces hastily took to flight to the waters of their side," it said. "The South Korean military authorities should make an apology to the north side for the armed provocation and take a responsible measure against the recurrence of the similar provocation." North Korea has intruded past the Northern Limit Line 22 times this year, but this was the first time the sides exchanged fire.